There is a clear demand for the monitoring of air-borne compounds that can have health effects on exposed individuals. A great interest exists for compounds that have occupational exposure limit values, set by governmental bodies, to ensure that the levels of such compounds are satisfactory low. In many cases, it is not known what the air contaminants consist of and for this reason, it is of interest to learn more details about the nature of these “unknown” compounds and to reveal the identity of the most predominate ones. Another field of interest is to study and check the effect of measures with a view to reducing these levels in air, e.g. to check the “true” ventilation efficiency or other measures to control the air levels. Devices for this purpose can also be used for the monitoring of the quality of compressed air and air in respiratory protective devices. Other fields of application for such devices are e.g. the control of different volatile compounds present in food. Such compounds can be used as markers for degradation of certain food components or to monitor raw materials to ensure a satisfactory quality. Such devices may also be used to ensure that other compounds have not contaminated to food. In hospitals, such devices can be used to check the air levels of e.g. narcosis gases and to ensure that the personnel, patients or others are not exposed to toxic levels. Chemical warfare agents are compounds that need to be checked for in order to reveal the presence thereof and to ensure that individuals are not exposed.
In environmental analysis there is a need to monitor the quality of air in cities, public places and in the nature. One purpose is to obtain background data for statistical studies and to check if the levels are below the levels set by national and international bodies. They can also be used to check if the emission of industrial pollutants results in exposure in the nature or in populated areas. The achieved data can have an impact on decisions and interpretation of a certain situation. There is therefore a demand of a satisfactory high quality of the data.
There are many examples of air pollutants that occur in both gas and particle phase. Of special interest are the size fractions that have the ability to reach the lower respiratory tract. There are reasons to believe that the toxicology is different depending on not only the chemistry as such but also on the distribution on different target organs in the body of humans. There is a need to know more about the exposure to the respirable particle fraction present in air.
Numerous devices exist for the monitoring of air-borne compounds and there is a great variety of technology used. In principle, the devices can be grouped in selective and non-selective devices. Non-selective devices give a response for several compounds and do not differentiate between two or several compounds and may also result in false positive results. Such devices are today still used, possibly due to the low cost. In many applications, false positive results can give rise to a high cost for the user, if costly measures are performed from invalid data.
Selective devices give a certain response for a selected compound or a group of compounds. Other present compounds do not interfere with the result. The frequency of false positive results will be much less as compared to non-selective monitoring. The quality of the data obtained is essential. Typical factors that describe the quality of the data are: repeatability, reproducibility, linearity (calibration graph characteristics with intercept and background), detection limit and quantification limit. In addition, knowledge regarding the interference from other compounds is necessary. It needs to be mentioned that a certain compound can influence the result even if the compound does not itself give rise to a response.
There are several drawbacks with the present types of instruments. For Photo Ionization Detector (PID) and Flame Ionization Detector (FID), identification of the individual chemicals is not possible. PID and FID detectors measure the sum of VOC (Volatile Organic Compounds). Infrared detectors suffer from problems with inferences. IR detectors are not possible to use when monitoring VOCs at low concentration when other interfering compounds are present.
For direct monitoring using GC-PID (e.g. VOC71M from Environment s.a.; www.environnementsa.com) and the GC-DMS instrument (e.g. Sionex Inc., Bedford, Mass., USA) there are limitations leading to inaccurate identification and quantification of analytes, and external complementary pre or post-calibration have to be made. For the existing products it is not possible to perform calibration automatically in the field. Further, there are problems with the occurrence of a non-linear relation between the sampling time and determined concentrations, which thereby disables long time sampling if the amount exceeds the calibration range. Further, when a volume is collected it needs to be calibrated to a volumetric volume and possibly corrected for the ambient temperature and air pressure. The sampling of a volume in a certain sampling volume container or on a sorbent followed by thermal desorption (in the case of a sorbent) and thereafter injecting the collected compounds on the GC the chromatographic peaks will be broadened in a way that the resolution of the chromatography will be affected.
A sampling device for analysis of air pollutants, more precisely polyurethane products, is disclosed in WO 00/75622, and further developments thereof are disclosed in WO 2011/108981 and in WO 2007/129965. The sampling devices, also called samplers, disclosed in these publications collect the probed chemical in a two-step process. A fluid in which the amount of a chemical is to be measured is pumped through the sampling device using a controlled flow. The chemical substance of interest present in the gas phase of the fluid is collected in an adsorption tube using a regent coated on the surfaces present inside the tube. The flow of fluid is further pumped from the adsorption tube to and through a filter impregnated with the same reagent. The chemical substance in solid form or adhered to particles in the fluid is collected in the filter.
An important parameter in this area is the gas flow containing the compound to detect, i.e. the analyte, in the apparatus used for the detection. During the sampling of compounds in air it is of importance to be able to control and log the flow and volume of the acquired amount of air through the sampling device as there is a direct correlation between the contents in a sample and the air volume collected. Taking several samples simultaneously is also of importance for three reasons, more precisely for increasing the accuracy of a certain sample, for detecting erroneous samples, and for acquiring different compounds simultaneously. When handling sampling results, it is also important to be able to track how the sample was collected, the time, the flow, the temperature, the pressure, and the humidity.
Existing solutions to maintain a stable flow during sampling do not prove to maintain a stable flow over time and require field calibration. The flow speed needs to be calibrated before and after sampling to ensure that the sampling speed is correct and have not changed over time. A logging functionality is also often missing.
Some existing solutions where a differential pressure sensor indicates if a change in the flow system back pressure has occurred, adjusts the pump control signal to compensate for this. However, this solution has proven to give drift errors over time, and a calibration with an external flow meter is required in order to set a certain flow rate of its pump.
Another existing solution has a logging function, an ability to transfer logged data to a PC, and an ability to control the flow via a display and buttons. Tests on such pumps did not concur with its specifications, as the pumps did not manage to keep a stable flow due to the fact that a sampler inducing a certain backpressure was attached to it.
A problem with existing pump systems is that the flow sensors incorporated in them may fluctuate with the temperature of the flow sensor electronics. Most flow sensors, using different techniques for the actual measurement of gas flow, have an output voltage signal corresponding to the measured flow. The output signal is however easily affected by the temperature of the electronic components in the flow sensor.
A further problem with the pumps for sampling purposes of the prior art is that the calibration of the pump mass flow sensor and thereby its measurement results is/are degraded relatively fast due to wear and damages to the sensors of the pump and to the pump engine. The pumps are often used in rough conditions at industrial work places and often outdoors.
In view of this, there is a great demand for an improved pump assembly for monitoring devices for the above mentioned detection of air-borne compounds, and for a pump that has the ability to deliver adequate pumping performance required for accurate measurements.
In that context there is further a demand for automatically detection of defects in sampling equipment e.g. in a pump assembly and a sampling device, or other devices used in sampling.